Shield adhesive having neutron shielding properties

ABSTRACT

An adhesive having shielding properties can provide a bonded part of a structure with neutron shielding properties. The shield adhesive having neutron shielding properties contains a lithium fluoride powder having lithium fluoride purity of 99% or more. The shield adhesive can be a two-pack curable adhesive and contain an epoxy resin as a main component. This shield adhesive can be a two-liquid curing type adhesive, which may contain an epoxy resin as a main component and may contain a modified silicone resin as a curing agent.

TECHNICAL FIELD

The present invention relates to a shield adhesive havingneutron-shielding performance.

BACKGROUND ART

In recent years, research and development of boron-neutron capturetherapy (BNCT) is rapidly progressing as a means of cancer treatment.Boron-neutron capture therapy is radiotherapy which uses a neutron beam.First, a boron compound that is specifically taken in by cancer cells isadministered to a patient. Subsequently, the cancer cells in which theboron compound has accumulated are irradiated with a neutron beam whoseenergy is controlled within a predetermined range. When the neutron beamcollides with the boron compound, α rays are generated. The cancer cellsare killed by the α rays.

Boron-neutron capture therapy is promising as a means of treating cancerand is moving into the clinical trials phase. The neutron irradiationapparatus used in boron-neutron capture therapy brings about therapeuticeffects by using a thermal neutron beam or an epithermal neutron beam.The neutron irradiation environment is a field where radioactive rayshaving energies in a certain range coexist.

It has been necessary so far to use a nuclear reactor as a neutrongenerator for supplying a neutron beam to a neutron irradiationapparatus. However, in recent years, small neutron generators to beinstalled in hospitals are being proposed. In such a small neutrongenerator, protons and deuterons accelerated by an accelerator collidewith a beryllium or lithium target. The generated neutron beam has ahigher proportion of thermal neutrons and epithermal neutrons than thosegenerated in conventional equipment. The generated neutron beam is thendecelerated by a moderator to provide a neutron beam irradiationenvironment having little influence on human bodies.

When neutrons are irradiated in neutron capture therapy, it is necessaryto provide a neutron-shielding means to irradiate a specific site. Inorder to examine the effects of neutron capture therapy, experiments ofirradiating small animals, such as mice, with neutrons have beenperformed. Patent Document 1 relates to a case of applying neutroncapture therapy using a shielding plate of lithium fluoride to mammalsother than human beings. The object thereof is to minimize neutronsgiven to normal tissue when the target site is deep inside theirradiation object, and to provide sufficient neutrons to the targetlocated deep inside the irradiation object by suppressing the reductionin depth and reachability of neutrons going into the body.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2004-233168

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in these shielding methods, the installability and shieldingproperties of a shield body may be restricted by the shape of theorganism which is to be an irradiation object, as well as the size ofthe irradiation site. Accordingly, there is required a shielding meansthat three-dimensionally shields the irradiation object and separates anirradiation region of the neutron beam from a non-irradiation region bymeans of a simple structure.

As the shielding means, a box-type structure having neutronbeam-shielding performance can be conceived, which can accommodate anorganism, i.e., an irradiation object. Such a box-type structure can beproduced by combining and joining a plurality of shielding plates. Inthe joining, an adhesive can be used. However, a neutron beam comingfrom the outside by an irradiation passes through the joining portionand enters the inside. Accordingly, it is desirable to impart a functionto shield the joining portion from neutrons. Thus, it is an object ofthe present invention to provide an adhesive having shielding propertiesthat can impart neutron-shielding properties to the joining portion of astructure having neutron-shielding performance.

Means for Solving the Problems

The present inventors have considered how to solve the above-describedproblems, and as a result, the present invention was accomplished bymeans of an adhesive containing a powder having a specific composition.Specifically, the present invention provides the following.

(1) The present invention relates to a shielding adhesive havingneutron-shielding performance, wherein the shielding adhesive contains alithium fluoride powder having a lithium fluoride purity of 99% or more.

(2) The present invention relates to the shielding adhesive according toaspect (1), wherein the lithium fluoride powder has an average particlesize of 2.4 μm or more and 5.2 μm or less.

(3) The present invention relates to the shielding adhesive according toaspect (2), wherein the shielding adhesive is a two-liquid curing typeadhesive of which a main component is an epoxy resin.

(4) The present invention relates to the shielding adhesive according toaspect (3), wherein the shielding adhesive contains lithium fluoridepowder in an amount of 30 wt % or more and less than 60 wt %.

(5) The present invention relates to the shielding adhesive according toaspect (2), wherein the shielding adhesive is a two-liquid curing typeadhesive containing an epoxy resin as a main component and a modifiedsilicone resin as a curing agent.

(6) The present invention relates to the shielding adhesive according toaspect (5), wherein the shielding adhesive contains lithium fluoridepowder in an amount of 30 wt % or more and 67 wt % or less.

(7) The present invention relates to the shielding adhesive according toany one of aspects (1) to (6), wherein the shielding adhesive is usedfor repairing a defective part of shielding plates made of a lithiumfluoride sintered body or for filling a gap at a joining portion of theshielding plates.

Effects of the Invention

A box-type structure having a simple three-dimensional structure can beprovided by applying a shielding adhesive according to the presentinvention for joining a plurality of shielding plates to each other.Since the shielding adhesive according to the present invention canimpart neutron-shielding to the site joined with the adhesive, theneutron-shielding performance of the entire structure can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating an embodiment of a box-typestructure to which a shielding adhesive is applied.

FIG. 2 includes schematic views illustrating another embodiment of abox-type structure to which a shielding adhesive is applied. (a) is afront view, and (b) is a perspective view.

FIG. 3 includes schematic views for explaining joining structures ofshielding plates to which a shielding adhesive is applied (a) is a viewillustrating a joining structure when the edge portions of shieldingplates have flat joining faces in a thickness direction. (b) is a viewillustrating a joining structure when the edge portions of shieldingplates have inclined joining faces. (c) is a view illustrating a joiningstructure when the edge portions of shielding plates have concave andconvex joining faces.

FIG. 4 includes views illustrating an exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 5 includes views illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 6 includes views illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 7 includes views illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 8 includes views illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 9 includes views illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied. (a) is a perspectiveview, (b) is a plan view, (c) is a cross-sectional view along the lineA-A, and (d) is a cross-sectional view along the line B-B.

FIG. 10 includes views illustrating another embodiment related to thebox-type structure to which a shielding adhesive is applied. (a) is aperspective view, (b) is a front view, (c) is a right-side view. (d) isa cross-sectional view along the line A-A, (e) is a cross-sectional viewalong the line B-B, (f) is a cross-sectional view along the line C-C,and (g) is a view illustrating the internal structure.

FIG. 11 includes views illustrating another embodiment related to thebox-type structure to which a shielding adhesive is applied. (a) is aperspective view, (b) is a perspective view from another direction, (c)is a front view, (d) is a plan view, (e) is a right-side view. (f) is across-sectional view along the line A-A, (g) is a cross-sectional viewalong the line B-B, and (h) is a cross-sectional view along the lineC-C.

FIG. 12 includes views illustrating another embodiment related to thebox-type structure to which a shielding adhesive is applied. (a) is aperspective view, and (b) is a perspective view from another direction.

FIG. 13 includes views illustrating another embodiment related to thebox-type structure to which a shielding adhesive is applied. (a) is aperspective view, and (b) is a perspective view from another direction.

FIG. 14 includes views illustrating another embodiment related to thebox-type structure to which a shielding adhesive is applied. (a) is aperspective view, and (b) is a perspective view from another direction.

FIG. 15 is a perspective view illustrating another embodiment related tothe box-type structure to which a shielding adhesive is applied.

FIG. 16 is a view illustrating another exemplary form of a shieldingplate to which a shielding adhesive is applied.

FIG. 17 includes schematic views for explaining joining structures ofshielding plates to which a shielding adhesive is applied. (a) is a viewillustrating a joining structure including edge portions having flatjoining faces. (b) is a view illustrating a joining structure includingedge portions having inclined joining surfaces. (c) is a viewillustrating a joining structure including edge portions having concaveand convex joining surfaces.

FIG. 18 includes views for explaining an embodiment of applying ashielding adhesive for repairing a shielding plate. (a) shows anexemplary application for repairing a corner of a shielding plate. (b)shows an examplary application for repairing a surface of a shieldingplate. (c) shows an examplary application for filling a gap of thejoining portion of shielding plates.

FIG. 19 is a view for explaining a box-type structure of an Example towhich a shielding adhesive is applied.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Embodiments of the shielding adhesive according to the present inventionwill now be described. The present invention is not limited to thefollowing embodiments and can be implemented by being appropriatelymodified within a range that does not deviate from the gist of thepresent invention.

(Shielding Adhesive)

The shielding adhesive according to the present invention is a shieldingadhesive having neutron-shielding performance. Edge portions ofshielding plates are preferably joined by the shielding adhesive. Thejoining structure of edge portions can be strengthened by applying theshielding adhesive to a face (end face) of the edge portions ofshielding plates and abutting the edge portions against each other. Inaddition, it is possible to temporarily fix a part of the end faces ofshielding plates with adhesive tape, and then fix permanently fix a partof the end faces with an adhesive. In this case, the temporarily fixedface can be used as a removable opening surface.

The shielding adhesive according to the present invention preferablycontains a lithium fluoride powder in an amount of 30 wt % or more. Suchan adhesive functions as a sealant filling the gap between the shieldingplates, as well as gules shielding plates together. The adhesiveincorporates lithium fluoride containing 6Li, which has aneutron-shielding function, has the effect of preventing a neutron beamfrom entering through the gap between the shielding plates.

When the viscosity of the adhesive is low, the adhesive may contain alithium fluoride powder in an amount of preferably 40 wt % or more, morepreferably 50 wt % or more, and further preferably 60 wt % or more inorder for adjusting the viscosity to have appropriate applicationworkability gap filling properties, and further enhancedneutron-shielding performance.

The shielding adhesive according to the present invention can containlithium fluoride in an adhesive, such as an epoxy resin or siliconeresin that does not contain, or contains a very small amount of, aninorganic filler. The known adhesive to be used is not particularlylimited, and an adhesive, such as an epoxy resin or a silicone resin,can be used. Since epoxy resin has a low content of elements other thancarbon, hydrogen, and oxygen, the radioactivation rate is low whenirradiated with a neutron beam. Accordingly, it is suitable for use inmedical fields such as radiotherapy. In addition, a two-liquid curingtype epoxy resin is cured by mixing a main agent and a curing agent.Accordingly, when a powder is added, the main agent and the curing agentcan be separately added to and kneaded with the powder and can be thenmixed for reaction and curing. Thus, the kneading time has a margin, andthe workability is improved. It is possible to select an adhesive havingan appropriate working time, curing time and viscosity. If the viscosityis too low, the adhesive is apt to flow out of the applied gap, and thegap filling property is reduced. Accordingly, it is preferable to use anadhesive having an appropriately high viscosity. A short curing time isproblematic because the time margin for assembling shielding plates intoa desired form is reduced. A long curing time is problematic because itrequires an elongated time for maintaining the shape of the structure,and therefore, temporary fixing must be performed for a long time.

(Box-Type Structure)

The box-type structure to which the shielding adhesive according to thepresent invention is applied includes a plurality of shielding platescontaining lithium fluoride, which has neutron-shielding performanceEdge portions of the shielding plates are abutted and joined to eachother. Here, the term “neutron shielding performance” refers to theperformance of shielding neutron beams. In the present specification,“neutron shielding” may be described as “neutron beam shielding”. Theshielding plate to which the shielding adhesive according to the presentinvention is applied means a neutron shielding plate. The box-typestructure is composed of m a plurality of shielding plates havingneutron shielding performance assembled into a box-like shape, and has asimple three-dimensional structure.

The external appearance of the box-type structure is shown in FIG. 1.Six shielding plates, e.g., shielding plates 1 and 2, are assembled tomake the box-type structure 10. The edge portion of the shielding platesare abutted on each other such that the end faces are closely joined toeach other by means of the shielding adhesive according to the presentinvention. The box-type structure shown in FIG. 1 is an embodiment inwhich all outer faces are surrounded by shielding plates.

The box-type structure 10 shown by (a) and (b) in FIG. 2 is anotherembodiment having an opening 20 provided in a part of the outer faces.Six shielding plates, e.g., shielding plates, 1, 2, 3, and 4, areassembled to make the box-type structure. The edge portions of theshielding plates are abutted on each other such that the end faces ofthe shielding plates are closely joined to each other. As shown by (a)and (b) in FIG. 2, a part of the sides of the shield 2 is short inlength such that the opening 20 is formed at that part.

The shielding plates of the box-type structure are made of a materialcontaining lithium fluoride (LiF), which is excellent in itsneutron-shielding performance. The shielding plates of the box-typestructure prevent the neutron beam radiated from the outside frompassing through. Accordingly, the shielding plates reduce the neutronbeams that reach the inside of the box-type structure, and thus can makea region (non-irradiation region) that is not substantially irradiatedwith the neutron beam in the box-type structure.

The box-type structure has a three-dimensional shape, such as a cube ora rectangular parallelepiped, which forms a three-dimensional neutronbeam-shielding space. The shielding space can accommodate a smallanimal, even if it has a certain volume or more.

Since the edge portions of the shielding plates are joined to each otherwith the shielding adhesive, a neutron beam is sufficiently preventedfrom passing through the gap between the edge portions. Accordingly, aneutron beam-shielding space can be formed inside the box-typestructure.

In the box-type structure having neutron-shielding performance, since aplurality of shielding plates having neutron-shielding performance arejoined in combination, a simple three-dimensional structure can beobtained. The size of the box-type structure can be freely adjustedaccording to the size and the number of the combined shielding plates.

In addition, the edge portions of the neutron-shielding plates can beeasily joined to each other by providing stepped or inclined joiningstructures at the edge portions of the neutron-shielding plates, andthus an effect of stabilizing the joining structure can be obtained.

It is possible to accommodate an organism or the like in a spaceshielded from a neutron beam and to perform a test in a state in whichnot all of the organism or the like is irradiated with a neutron beam.Furthermore, when the box-type structure has a partly open structure, apart of the organism or the like can be taken outside of the shieldingregion and can be irradiated with a neutron beam in only that part,broadening the application range of neutron beams in irradiation objectsand test conditions.

This box-type structure is suitable for the field of radiating a neutronbeam and examining its action and influence. It can be used in a testusing a small animal such as a mouse, or in an irradiation test relatingto radiotherapy, etc. In addition, a plurality of irradiation bodies canbe simultaneously irradiated by providing, for example, partitions inthe box-type structure or by providing a plurality of the box-typestructures, which improves the efficiency of an irradiation test.

(Shielding Plates)

The shielding adhesive according to the present invention can be appliedto shielding plates 22 and 23 including edge portions having flatjoining faces in the thickness direction as shown by (a) in FIG. 3. Inaddition, the shielding adhesive can be applied to shielding plates 24and 25 including edge portions having inclined joining faces as shown by(b) in FIG. 3. In these shielding plates, the edge portions are abuttedagainst each other, and the joining surfaces are fixed to each otherwith a shielding adhesive. Accordingly, firm engagement and sufficientneutron-shielding performance are imparted to the assembly of theshielding plates.

Furthermore, the shielding adhesive can be applied to the shieldingplates 26 and 27 including edge portions having stepped joining faces asshown by (c) in FIG. 3. A concave portion (hereinafter, referred to as“concave”) or a convex portion (hereinafter, referred to as “convex”) isprovided at the edge portions of the shielding plates 26 and 27 in astepped form. The concave and the convex make a close fit together. Thejoining faces are fixed with a shielding adhesive. Regarding the joiningstructure in which a shielding adhesive is applied to a shielding platehaving such a stepped joining surface, there are various exemplary formsas shown in FIGS. 4 to 9.

(a) to (c) in FIG. 17 show exemplary forms in which joining structuresof box-type structures are formed by using these shielding plates. (a)in FIG. 17 is a view illustrating a joining structure in which ashielding adhesive is applied to shielding plates having flat joiningfaces. (b) in FIG. 17 is a view illustrating a joining structure inwhich a shielding adhesive is applied to shielding plates havinginclined joining faces. (c) in FIG. 17 is a view illustrating a joiningstructure in which a shielding adhesive is applied to joining faceshaving stepped concave and convex portions.

The shielding adhesive according to the present invention can be appliedfor repair when a part of a shielding plate is chipped off. For example,(a) in FIG. 18 shows an example of a shield body having a chippedportion 29 at a corner of a shielding plate 28. (b) in FIG. 18 shows anexample of a shield body having a chipped portion 29 on a surface of ashielding plate 28. In each of the shield bodies 28, the chipped offportion or a part having the same shape as that of the chipped offportion is joined to the body of the shielding plate by means of ashielding adhesive to restore the shielding plate to the original shape.(c) in FIG. 18 shows an example of using the shielding adhesive incombination with a neutron beam sealant as a filler for structuralstrengthening when the gap between the halving joint portions of theshielding plates is relatively large.

(a) to (d) in FIG. 4 are views illustrating exemplary forms relating toshielding plates to which a shielding adhesive according to the presentinvention is applied. (a) in FIG. 4 is a perspective view, (b) in FIG. 4is a plan view, (c) in FIG. 4 is a cross-sectional view along the lineA-A, and (d) in FIG. 4 is a cross-sectional view along the line B-B.FIGS. 5 to 9 show other exemplary forms of shielding plates. In eachfigure, similar to (a) to (d) of FIG. 4, (a) is a perspective view, (b)is a plan view, (c) is a cross-sectional view along the line A-A, and(d) is a cross-sectional view along the line B-B.

FIG. 4 shows an example of a shielding plate including edge portionshaving stepped joining faces at four sides of the shielding plate. Asunderstood from the cross-sectional views shown by (c) and (d) in FIG.4, the lengths of the four sides of the shielding plate on the upperside 31 are all shorter in length than those of the four sides on thelower side 32. As a result, as shown by (a) in FIG. 4, this shieldingplate has a shape including steps 33 formed at the edge portions of foursides.

FIG. 5 shows another example of a shielding plate including edgeportions having stepped joining faces at four sides of the shieldingplate. As understood from the cross-sectional views shown by (c) and (d)in FIG. 5, the upper side and the lower side of the shielding platediffer from each other in the lengths of the four sides. The two sides31S among the four sides on the upper side 31 are shorter than thecorresponding two sides on the lower side 32, while the other two sides31L on the upper side are longer than the corresponding two sides on thelower side 32. As a result, as shown by (a) in FIG. 5, this shieldingplate has a shape including steps 33 formed at the edge portions of foursides.

FIG. 6 shows an example of a shielding plate including edge portionshaving stepped joining faces at three sides of the shielding plate. Asunderstood from the cross-sectional views shown by (c) and (d) in FIG.6, three sides among the four sides on the upper side of the shieldingplate are shorter in length than the corresponding three sides on thelower side. As a result, as shown by (a) in FIG. 6, this shielding platehas a shape including steps formed at the edge portions of the threesides.

FIG. 7 shows an example of a shielding plate including edge portionshaving stepped joining faces at two sides of the shielding plate. Asunderstood from the cross-sectional views shown by (c) and (d) in FIG.7, two sides among the four sides on the upper side 31 of a shieldingplate are shorter in length than the corresponding two sides on thelower side 32. As a result, as shown by (a) in FIG. 7, this shieldingplate has a shape including steps 33 formed at the edge portions of twosides.

FIG. 8 shows another example of a shielding plate including edgeportions having stepped joining faces at two sides of the shieldingplate. As understood from the cross-sectional views shown by (c) and (d)in FIG. 8, the four sides on each of the upper side 31 and the lowerside 32 of the shielding plate have the same length. However, two sidesamong the four sides are arranged at different positions from thecorresponding two sides. As a result, as shown by (a) in FIG. 8, thisshielding plate has a shape including steps 33 formed at the edgeportions of two sides.

FIG. 9 shows an example of a shielding plate including edge portionshaving a stepped joining face at one side of the shielding plate. Asunderstood from the cross-sectional views shown by (c) and (d) in FIG.9, one side among the four sides on the upper side 31 of a shieldingplate is shorter in length than the corresponding one side on the lowerside 32. As a result, as shown by (a) in FIG. 9, this shielding platehas a shape including a step 33 formed at the edge portion of one side.

The shielding plates are not limited to those including theabove-described edge portions having stepped joining surfaces. Theshielding plate may include an edge portion having an inclined joiningface as shown by (b) in FIG. 3 and may similarly have a shape includingan inclined joining face formed at each edge portion of four sides,three sides, two sides, or one side.

The planar shapes of the shielding plates shown in FIGS. 4 to 9 are eacha square, but the shape is not limited thereto and may be a rectangle asshown in FIG. 16.

A neutron beam radiated from the outside may pass through the shieldingplates and the joining portion. For example, when the joining portion asshown by (a) in FIG. 3 includes an even joining surface, the neutronbeam that has entered the joining portion may pass therethrough andenter the inside of the structure. When the joining portion includesinclined or concave and convex joining faces as shown by (b) or (c) inFIG. 3, the neutron beam that has passed through the shield body maypass through the joining portion and enter the inside. In contrast, theshielding adhesive according to the present invention imparts sufficientneutron-shielding properties to the joining portion. Accordingly, it ispossible to prevent the neutron beam from passing through the joiningportion, and to obtain an effect of improved shielding properties of theentire structure composed of shielding plates.

In addition, the structure composed of shields is stably fixed byjoining the edge portions of the shielding plates with a shieldingadhesive, causing an effect of increasing mechanical strength.

(Face of Box-Type Structure)

The box-type structure to which the shielding adhesive according to thepresent invention is applied has a plurality of faces, and at least oneof the faces is preferably removable. The box-type structure isthree-dimensionally configured and therefore has a plurality of outerfaces. These faces have a removable structure. Accordingly, it isconvenient for disassembling the box-type structure in, for example,preparation for an irradiation test or withdrawal after the test. Inaddition, the organism, i.e., an irradiation object, can be easily putin and taken out.

The box-type structure can be provided with an opening portion in a partof the faces. Since the outside of the box-type structure is in anenvironment under irradiation with a neutron beam, it is possible toexpose a part of the irradiation object to the outside through theopening portion and to irradiate the part exclusively. For example, inorder to irradiate the leg of a mouse exclusively for a test with aneutron beam, it is possible to three-dimensionally cover the mouse bodywith the box-type structure and place only the leg of the mouse in theirradiation environment outside the box-type structure.

(Joining with Adhesive Tape)

The shielding plates may be fixed with adhesive tape in addition to thejoining with a shielding adhesive. Alternatively, the edge portions ofthe shielding plates are abutted against each other, and the shieldingplate surfaces are then attached to adhesive tape to fix the shieldingplates. Thus, a box-type structure can be temporarily assembled.

The box-type structure fixed with adhesive tape can be disassembled bypeeling off the adhesive tape after being used in a necessary test orcan be reassembled. The adhesive tape may be any known product withoutspecific limitation. In order to minimize the radioactivation rate byirradiation with a neutron beam, it is preferable to select colorlesstransparent or translucent tape free from coloring components andinorganic fillers in the tape base. The adhesive component adhering tothe tape base surface is not particularly limited and may be, forexample, a rubber or acrylic adhesive agent. The material of the tapebase is not particularly limited and may be, for example, cellophane oracetate.

(Embodiment of Box-Type Structure)

The box-type structure to which a shielding adhesive according to thepresent invention is applied can be provided in various shapes andconfigurations by combining the above-described shielding plates.Exemplary forms thereof are shown in FIGS. 10 to 15.

(a) to (g) in FIG. 10 show embodiments related to the box-typestructure. (a) in FIG. 10 is a perspective view, (b) in FIG. 10 is afront view, and (c) in FIG. 10 is a right-side view. (d) in FIG. 10 is across-sectional view along the line A-A, (e) in FIG. 10 is across-sectional view along the line B-B, and (f) in FIG. 10 is across-sectional view along the line C-C. (g) in FIG. 10 is a viewillustrating the inside of the box-type structure when the uppershielding plate 46 is detached. As shown by (a) to (c) in FIG. 10, it ispossible to provide a box-type structure composed of a combination ofsix shielding plates 41, 42, 43, 44, 45, and 46 that has a structure inwhich the shielding plates enclose the internal space completely. Theshielding plates 41, 42, 43, 44, 45, and 46 have a structure includingthe edge portions abutted and joined via the joining portions. On theinside of the box-type structure, the stepped joining structure has aconcave on the inside, as shown by (b) in FIG. 10.

(a) to (h) in FIG. 11 show another embodiment related to the box-typestructure to which a shielding adhesive is applied. (a) in FIG. 11 is aperspective view, (b) in FIG. 11 is a perspective view from anotherdirection, and (c) in FIG. 11 is a front view. (d) in FIG. 11 is a planview, and (e) in FIG. 11 is a right-side view. (f) in FIG. 11 is across-sectional view along the line A-A, (g) in FIG. 11 is across-sectional view along the line B-B, and (h) in FIG. 11 is across-sectional view along the line C-C. As shown in (a) to (e) in FIG.11, the box-type structure is composed of a combination of fiveshielding plates 41, 42, 43, 44, and 45. As shown by (f) to (h) in FIG.11, these shielding plates are abutted and joined to each other via ajoining structure formed at the edge portions. One opening is providedat the upper portion of the box-type structure to give a structure inwhich the joining portions at the edge portions of the shielding platesare arranged to protrude outwards along the periphery of the opening.Accordingly, covering the opening with another shielding plate makes itpossible to give a joining structure including end faces contacting eachother closely. In addition, it is possible to configure a rectangularparallelepiped box-type structure by combining the box-type structurewith another box-type structure through connecting the openings to eachother. A shielding plate may be attached to the opening so as to close apart of the opening.

(a) and (b) in FIG. 12 show another embodiment related to the box-typestructure to which a shielding adhesive is applied. (a) in FIG. 12 is aperspective view, and (b) in FIG. 12 is a perspective view from anotherdirection. As shown by (a) and (b) in FIG. 12, this is an exampleincluding an opening provided at each of the upper side and the lowerside of the box-type opening portion. Similar to the box-type structurein FIG. 10, there is provided a structure in which the joining portionsat the edge portions of the shielding plates are arranged to protrudeoutward along the peripheries of the openings. Except for the above,similar to the box-type structure in FIG. 10, the shielding plates 41,42, 43, and 44 have a structure including the edge portions are abuttedand joined via the joining portions.

(a) and (b) in FIG. 13 are views illustrating another embodiment relatedto the box-type structure to which a shielding adhesive is applied. (a)in FIG. 13 is a perspective view, and (b) in FIG. 13 is a perspectiveview from another direction. As shown by (a) and (b) in FIG. 13, this isan example including an opening provided at each of the upper side andthe lower side of the box-type opening portion. Similar to the box-typestructure in FIG. 11, there is provided a structure in which the joiningportions of the edge portions of the shielding plates are arranged toface inward along the peripheries of the openings. Except for the above,similar to the box-type structure in FIG. 10, the shielding plates 41,42, 43, and 44 have a structure including the edge portions abutted andjoined via the joining portions is provided.

(a) and (b) in FIG. 14 show another embodiment related to the box-typestructure to which a shielding adhesive is applied. (a) in FIG. 14 is aperspective view, and (b) in FIG. 14 is a perspective view from anotherdirection. As shown by (a) and (b) in FIG. 14, this is an exampleincluding an opening provided at each the upper side and the lower sideof the box-type opening portion. Similar to the box-type structure inFIG. 10, there is provided a structure in which the joining portions atthe edge portions of the shielding plates are arranged to protrudeoutward along the periphery of the opening at the upper side. Similar tothe box-type structure in FIG. 11, there is provided a structure inwhich the joining portions at the edge portions of the shielding platesare arranged to face inward along the periphery of the opening at thelower side. Except for the above, similar to the box-type structure inFIG. 10, the shielding plates 41, 42, 43, and 44 have a structureincluding the edge portions abutted and joined via the joining portions.

The shielding plates shown in FIGS. 10 to 14 each have a square planeshape, but the shape is not limited thereto and may be a rectangle asshown in FIG. 15.

(Lithium Fluoride Powder Material and a Lithium Fluoride Sintered Body)

Regarding a material containing lithium fluoride (LiF), Li includes twostable isotopes ⁶Li and ⁷Li. The natural abundance ratios of ⁷Li and ⁶Liare 92.5 atom % and 7.5 atom %, respectively. Since ⁶Li contributes tothe shielding of a neutron beam, the neutron beam can be moreefficiently shielded by using ⁶LiF containing ⁶Li. The shielding platemade of a lithium fluoride-containing material is preferably a lithiumfluoride sintered body prepared by shaping and sintering a lithiumfluoride powder to obtain an edge portion structure having apredetermined shape. For the material of the shielding plate accordingto the present embodiment, the content of ⁶Li can be adjusted accordingto the necessary neutron-shielding performance. For example, when highneutron-shielding performance is required, it is possible to select alithium fluoride powder material having a ⁶Li content of 95 atom % and aLiF purity of 99% or more. Alternatively, it is also possible to uselithium fluoride having an appropriately adjusted content ratio of ⁶Liand ⁷Li.

A lithium fluoride sintered body including ⁶LiF can be obtained withoutadding a sintering agent or other inorganic compound to form a compositematerial. Accordingly, the shielding plate made of lithium fluorideaccording to the present embodiment can have excellent neutron-shieldingperformance due to the high purity of the lithium fluoride itself.

(Lithium Fluoride Powder Material Used in Shielding Adhesive)

The lithium fluoride powder material used in the shielding adhesive canbe an equivalent to the lithium fluoride powder used for sintering ofthe shielding plates. The use of such a lithium fluoride powder allowsshielding performance to be obtained with a smaller amount thereof. Forexample, it is preferable to use a lithium fluoride powder having a ⁶Licontent of 95 atom % and a LiF purity of 99% or more.

For the lithium fluoride-containing material according to the presentembodiment, the purity of LiF is preferably 99 wt % or more. If theshield material contains a large amount of impurities, the impuritiesirradiated with a neutron beam might be radiaoactivated to emit gammarays. LiF itself is not radioactivated even if irradiated with a neutronbeam. Accordingly, regarding the lithium fluoride-containing materialaccording to the present embodiment, if the purity of the lithiumfluoride itself is low or a sintering agent or other inorganic compound,i.e., a composite material, is mixed, such impurities or mixedcomponents may be radioactivated and emit gamma rays. Thus, the use oflithium fluoride having a high purity is desirable.

Examples of methods for manufacturing a lithium fluoride product includea single crystal growth method, a solidifying method from a melt, and asintering method. However, the sintering method is preferable becausethis method makes it possibly to supply a stable-quality product at alow cost.

The single crystal growth method requires high control accuracy duringmanufacturing, resulting in low quality stability and a significantlyhigh product price. In addition, the resulting single-crystal body hasproblems such as a high cost for processing into a predetermined shape,cleavability, and the easy occurrence of cracks during processing. Thesolidifying method from a melt requires strict temperature controlduring cooling and requires cooling for a long time, resulting indifficulty in obtaining a solid substance having a relatively large sizeand is homogeneous and defect-free throughout the substance.

The sintered body of lithium fluoride (hereinafter may be also referredto as “LiF sintered body”) preferably has a relative density of 86% ormore and 92% or less. In the present embodiment, the relative density isa value obtained by dividing the density of a sintered body by thetheoretical density of LiF (2.64 g/cm³) and multiplying the result by100. The lithium fluoride sintered body having a relative density withinthe above range has the advantage that swelling and the occurrence ofvoids and cracks during sintering are suppressed to provide excellentmachinability. The LiF sintered body is not highly-densified, resultingin the advantage of excellent machinability.

If the relative density is too low, there is a risk that the LiFsintered body does not have sufficient neutron-shielding performance. Inaddition, if the relative density is too low, there is a concern thatthe ratio of voids inside the sintered body is high, resulting in aninferior mechanical strength.

In contrast, if the relative density is too high, the LiF sintered bodyhas sufficient neutron-shielding performance. However, there is aconcern that the sintered body is highly densified such that theprocessing of the sintered body may cause cracks or the like due to areleased residual stress inside the material.

The thickness of the LiF sintered body is not particularly limited aslong as a neutron beam can be suitably blocked. Specifically, thethickness of the LiF sintered body is preferably 2 mm or more and morepreferably 3 mm or more from the viewpoint of the mechanical strength ofthe sintered body and workability during edge portion processing.

The upper limit of the thickness of the LiF sintered body is notparticularly limited. From the viewpoint of reducing the size and weightof the shielding plate, a thinner LiF sintered body is preferred withina range capable of suitably shielding a neutron beam. Specifically, thethickness of the LiF sintered body is preferably 8 mm or less and morepreferably 5 mm or less.

(Method for Manufacturing LiF Sintered Body)

The method for manufacturing the LiF sintered body according to thepresent embodiment includes a pressing step of pressing a LiFcomposition containing a LiF powder and an organic shaping agent toprepare a pressed body, and a firing step of firing the pressed body at630° C. or more and 830° C. or less. Prior to the firing step, forexample, a preliminary firing step for degreasing the organic shapingagent may be performed.

EXAMPLES

The present invention will now be described in more detail usingexamples. The present invention is not limited to these descriptions.

(Manufacturing of Box-Type Structure)

Base plates of a lithium fluoride (LiF) sintered body having a length of80 mm, a width of 40 mm, and a thickness of 5 mm were produced. Thissintered body had a relative density within a range of 88.9% to 91.3%. Ashielding plate 1A and a shielding plate 2A each having an edge portionstructure with a predetermined shape were produced by performing edgeportion processing for forming a step having a length of about 2.5 mmalong the peripheries of the base plates. The shielding plates 1A and 2Awere each provided with a step at each of the three peripheral edges. Inaddition, base plates having a length of 80 mm, a width of 40 mm, and athickness of 5 mm were produced and subjected to edge portion processingfor forming a step having a length of about 2.5 mm at each of the fourperipheral edges to produce shielding plates 3A and 3B. Similarly, abase plate having a length of 80 mm, a width of 45 mm, and a thicknessof 5 mm was produced and subjected to edge portion processing to form astep having a length of about 2.5 mm at each of the four edges toproduce a shielding plate 4A. Furthermore, a base plate having a lengthof 80 mm, a width of 25 mm, and a thickness of 5 mm was similarlyproduced and subjected to edge portion processing to form a step havinga length of about 2.5 mm at each of the three sides to produce ashielding plate 4B.

Subsequently, the shielding plate 1A and the shielding plate 2A wereabutted against each other such that two were joined to each other atthe edge portions of the side faces where no steps were provided in thelongitudinal direction, to produce a shielding plate 1B and a shieldingplate 2B each having a length of 80 mm and a width of 80 mm. In theabutting and joining, a shielding adhesive was used for fixation. Theseshielding plates 1B, 2B, 3A, 3B, 4A, and 4B were combined to assemble abox-type structure as shown in FIG. 19.

(Adhesive Containing Lithium Fluoride)

A lithium fluoride powder (LiF powder) having a neutron-shieldingfunction with a ⁶Li content of 95 atom % was mixed with commerciallyavailable adhesives to prepare shielding adhesive samples, andperformance of the shielding adhesives was verified. The types of theadhesives used (S1 to S5) and the mixing ratios (mass ratios) of the LiFpowder and each adhesive were as shown in Tables 1 to 3. Table 1 showsdata for the case where a LiF powder having an average particle size of3.8 μm was used. Table 2 shows data for the case where a LiF powderhaving an average particle size of 2.4 μm was used. Table 3 shows datafor the case where a LiF powder having an average particle size of 5.2μm was used.

Two test plates each having a length of 40 mm, a width of 20 mm, and athickness of 5 mm and made of a LiF sintered body were prepared. Theedge portions of the test plates were flat and did not have steps. About30 mg of an adhesive for the test was applied to the 40 mm length endfaces of the test plates, and the test plates were abutted and joined toeach other. The excess adhesive overflowing from the gap was removed,followed by temporary fixing and being left to cure until it reached thepractical strength described in the respective instruction manuals.

The performance of each adhesive was evaluated from the viewpoints ofkneadability, workability, physical properties after curing (stiffnessand heat resistance), and shielding properties as shown in Tables 1 to3. Then, the performance was ranked in accordance with each evaluationcriteria of good (A), moderate (B), and unsuitable (C) according to thedegree thereof. The results are shown in Tables 1 to 3 with the symbols“A”, “B”, and “C”. The symbol “-” in Tables 1 to 3 indicatesperformances that were not evaluated. Samples that were rated C forkneadability or workability were not evaluated for physical propertiesafter curing.

Kneadability is a performance to evaluate the degree of uniformity inthe paste-like mixture prepared by kneading an adhesive and a LiF powderat a predetermined mixing ration. For Kneadability, a lower viscosity ofthe adhesive or a lower mixing ratio of the LiF powder/adhesive wasdetermined to be better.

<Method for Measuring Average Particle Size>

The particle size distribution of the LiF powder was measured using aMicrotrac MT3300 manufactured by Nikkiso Co., Ltd. under the followingconditions: particle refractive index: 1.39, solvent (water) refractiveindex: 1.33, particle permeability: transmission, particle shape:non-spherical, external dispersion: none, and internal dispersion: 0 to3 minutes. The 50% accumulation diameter μm obtained by measurement wasdefined as the average particle size.

<Kneading Method>

In the case of a one-liquid curing type adhesive, a predetermined amount(200 mg) of the adhesive was weighed on a resin sheet using amicrospatula. A predetermined amount (200 mg) of the lithium fluoridepowder separately-weighed using a medicine wrapping paper wastransferred to a spot near the dropped adhesive, and they were thenkneaded with a small resin spatula until become uniform. The kneadedproduct was applied to the joining portion of a lithium fluoridesintered body.

In the case of a two-liquid curing type adhesive, a predetermined amount(100 mg) of each of the adhesive main agent and the adhesive curingagent were separately dropped onto a resin sheet at an interval ofseveral cm using a microspatula. A predetermined amount (100 mg) oflithium fluoride powder separately-weighed using medicine wrapping paperwas transferred to a spot near the adhesive main agent and a spot nearthe adhesive curing agent. Subsequently, the mixture of the adhesivemain agent and the lithium fluoride powder and the mixture of theadhesive curing agent and the lithium fluoride powder were thenseparately kneaded using a spatula similar to the above until becomeuniform. Then, the mixtures were combined and kneaded together using aspatula similar to the above until becomes uniform. The kneaded productwas applied to the joining portion of a lithium fluoride sintered body.

Workability was evaluated according to the balance between appropriateworking time and curing time When the LiF powder and the adhesive arekneaded, if the curing of the adhesive proceeds too quickly, uniformmixing is difficult. In contrast, if the adhesive has a low viscosityand takes a long time to cure, it is necessary to fix the joiningmaterial with a jig or the like for a long time. In addition, fillingproperty and shape retention property were also considered from the viewpoint of the degree how easily the mixture fills the gap at the joiningsite and how much the mixture flow out to reduce.

The physical properties after curing were evaluated for stiffness andheat resistance. Regarding the stiffness, test pieces having a length of40 mm, a width of 5 mm, and thickness of 5 mm were each prepared bycutting a joined sintered body in a direction perpendicular to thejoining line into a strip having a width of about 5 mm. A force wasapplied with a hand so as to fold the test piece in order to verify thepresence or absence of interfacial peeling or cohesive fractures at thejoining portion and the presence or absence of fractures at the sinteredbody portion. A test piece that was fractured at the sintered bodyportion was rated ‘A’. A test piece that was fractured at the joiningportion, although it would not cause a problem in normal use, was ratedas ‘B’.

Regarding heat resistance, the test pieces were heated at 100° C. for 1hour with an electric dryer. A test piece wherein the joining portionwas not deformed by heating and the strength before heating was mostlymaintained was rated ‘A’.

Regarding shielding properties, since the required shielding propertiesvary depending on the purpose, the amounts of LiF contained in theshielding adhesives used were listed as relative values in Table 1.

The results of evaluation from the above-described viewpoints are shownin Tables 1 to 3. Table 1 shows the results for the case where a LiFpowder having an average particle size of 3.8 μm was used. As shown inTable 1, it was understood that the two-liquid curing type epoxyadhesives used in sample Nos. 1 to 3 and 5 to 7 were preferable whencomprehensively evaluated in regard to kneadability, workability, andphysical properties after curing. It can be said that when theapplication area to which a shielding adhesive is applied is small, afast curing type of two-liquid curing type epoxy is preferred, and thatwhen the application number and the application area are large, atwo-liquid curing type epoxy is preferred.

In addition, even if the mixing ratio of the LiF powder was 30% or more,satisfactory results were obtained. Samples Nos. 9 to 13 are exampleswherein a two-liquid type epoxy/modified silicone adhesive was used. Thekneadability and the balance between working time and curing time inworkability were good. However, under conditions of a low mixing ratio,since the viscosity was low, the filling and shape retention propertieswere somewhat insufficient, and the stiffness after curing was alsosomewhat inferior compared to Nos. 1 to 3 and 5 to 7.

The one-liquid type modified silicone used in sample Nos. 15 and 16 wasinadequate in its kneadability and workability. Accordingly, thephysical properties after joining were not evaluated.

Table 2 shows the results for the case where a LiF powder having anaverage particle size of 2.4 μm was used. As shown in Table 2, it wasunderstood that the two-liquid curing type epoxy adhesives used insample Nos. 1 to 3 and 5 to 7 were preferable when comprehensivelyevaluated in regard to kneadability, workability, and physicalproperties after curing. It can be said that when the application areato which a shielding adhesive is applied is small, a fast curing type oftwo-liquid curing type epoxy is preferred, and that when the applicationnumber and the application area are large, a two-liquid curing typeepoxy is preferred.

In addition, even if the mixing ratio of the LiF powder was 30% or more,satisfactory results were obtained. Sample Nos. 9 to 13 are examples ofusing a two-liquid type epoxy/modified silicone adhesive. Thekneadability and the balance between working time and curing time in theworkability were good. However, under conditions of a low mixing ratio,since the viscosity was low, the filling and shape retention propertieswere somewhat insufficient, and the stiffness after curing was alsosomewhat inferior compared to Nos. 1 to 3 and 5 to 7.

Since the one-liquid type modified silicone used in sample Nos. 15 and16 was inadequate in its kneadability and workability, the physicalproperties after joining were not evaluated.

Table 3 shows the results for the case where a LiF powder having anaverage particle size of 5.2 μm was used. As shown in Table 3, it wasunderstood that the two-liquid curing type epoxy adhesives used insample Nos. 1 to 4 and 6 to 9 were preferable when comprehensivelyevaluated in regard to kneadability, workability, and physicalproperties after curing. It can be said that when the application areato which a shielding adhesive is applied is small, a fast curing andtwo-liquid curing type epoxy is preferred, and that when the applicationnumber and the application area are large, a two-liquid curing typeepoxy is preferred.

In addition, even if the mixing ratio of the LiF powder was 30% or more,satisfactory results were obtained. Sample Nos. 11 to 15 are exampleswherein a two-liquid type epoxy/modified silicone adhesive was used. Thekneadability and the balance between working time and curing time inworkability were good. However, under conditions of a low mixing ratio,since the viscosity was low, the filling and shape retention propertieswere somewhat insufficient, and the stiffness after curing was alsosomewhat inferior compared to Nos. 1 to 3 and 6 to 9.

Since the one-liquid type modified silicone used in sample Nos. 17 and18 was inadequate in its kneadability and workability, the physicalproperties after joining were not evaluated.

TABLE 1 Performance of adhesive Workability Working Filling Physicalproperties LiF/adhesive time/ and shape after curing Shielding Adhesivemixing ratio curing retention Heat performance Sample Product Type (massratio) Kneadability time properties Stiffness resistance LiF wt %  1 S1Two-liquid 20/80 A A B A A 20  2 curing 30/70 A A A A A 30  3 type epoxy50/50 B A A A A 50  4 60/40 C C — — — —  5 S2 Two-liquid 20/80 A A B A A20  6 curing type 30/70 A A A A A 30  7 epoxy Fast 50/50 A B A A A 50  8curing type 60/40 C C — — — —  9 S3 Two-liquid 20/80 A A C B A 20 10curing type 30/70 A A B B A 30 11 epoxy 50/50 A A A A A 50 12 Epoxyresin/ 60/40 A A A A A 60 13 modified 67/33 B B A A A 67 14 silicone70/30 C C — — — — 15 S4 One-liquid 30/70 C C — — — — type specificmodified silicone 16 S5 One-liquid 30/70 C C — — — — type specificmodified silicone Fast curing type S1: Araldite (registered trademark)Standard (manufactured by Huntsman Japan KK, two-liquid curing typeepoxy adhesive, epoxy resin/modified polyamine) S2: Araldite (registeredtrademark) Rapid (manufactured by Huntsman Japan KK, two-liquid curingtype epoxy adhesive, fast curing type, epoxy resin/modified polythiol)S3: EP001N (manufactured by Cemedine Co., Ltd., two-liquid curing typeepoxy adhesive, epoxy resin/modified silicone) S4: Super X Clear(manufactured by Cemedine Co., Ltd., one-liquid type specific modifiedsilicone polymer) S5: Super X2 Clear (manufactured by Cemedine Co.,Ltd., one-liquid type specific modified silicone polymer)

TABLE 2 Performance of adhesive Workability Working Filling Physicalproperties LiF/adhesive time/ and shape after curing Adhesive mixingratio curing curing Heat Sample Product Type (mass ratio) Kneadabilitytime properties Stiffness resistance  1 S1 Two-liquid 20/80 A A B A A  2curing 30/70 A A A A A  3 type epoxy 50/50 B B A A A  4 60/40 C C — — — 5 S2 Two-liquid 20/80 A A B A A  6 curing type 30/70 A A A A A  7 epoxyFast 50/50 A B A A A  8 curing type 60/40 C C — — —  9 S3 Two-liquid20/80 A A C B A 10 curing type 30/70 A A B B A 11 epoxy 50/50 A A A A A12 Epoxy resin/ 60/40 A A A A A 13 modified 67/33 B B A A A 14 silicone70/30 C C — — — 15 S4 One-liquid 30/70 C C — — — type specific modifiedsilicone 16 S5 One-liquid 30/70 C C — — — type specific modifiedsilicone Fast curing type S1: Araldite (registered trademark) Standard(manufactured by Huntsman Japan KK, two-liquid curing type epoxyadhesive, epoxy resin/modified polyamine) S2: Araldite (registeredtrademark) Rapid (manufactured by Huntsman Japan KK, two-liquid curingtype epoxy adhesive, fast curing type, epoxy resin/modified polythiol)S3: EP001N (manufactured by Cemedine Co., Ltd., two-liquid curing typeepoxy adhesive, epoxy resin/modified silicone) S4: Super X Clear(manufactured by Cemedine Co., Ltd., one-liquid type specific modifiedsilicone polymer) S5: Super X2 Clear (manufactured by Cemedine Co.,Ltd., one-liquid type specific modified silicone polymer)

TABLE 3 Performance of adhesive Workability Working Filling Physicalproperties LiF/adhesive time/ and shape after curing Adhesive mixingratio curing curing Heat Sample Product Type (mass ratio) Kneadabilitytime properties Stiffness resistance  1 S1 Two-liquid 20/80 A A B A A  2curing type 30/70 A A A A A  3 epoxy 50/50 B A A A A  4 60/40 B B A A A 5 67/33 C C — — —  6 S2 Two-liquid 20/80 A A A A A  7 curing type 30/70A A A A A  8 epoxy 50/50 A A A A A  9 Fast curing 60/40 A B A A A 10type 67/33 C C — — — 11 S3 Two-liquid 20/80 A A C B A 12 curing 30/70 AA B B A 13 type epoxy 50/50 A A A A A 14 Epoxy resin/ 60/40 A A A A A 15modified 67/33 B B A A A 16 silicone 70/30 C C — — — 17 S4 One-liquid30/70 C C — — — type specific modified silicone 18 S5 One-liquid 30/70 CC — — — type specific modified silicone Fast curing type S1: Araldite(registered trademark) Standard (manufactured by Huntsman Japan KK,two-liquid curing type epoxy adhesive, epoxy resin/modified polyamine)S2: Araldite (registered trademark) Rapid (manufactured by HuntsmanJapan KK, two-liquid curing type epoxy adhesive, fast curing type, epoxyresin/modified polythiol) S3: EP001N (manufactured by Cemedine Co.,Ltd., two-liquid curing type epoxy adhesive, epoxy resin/modifiedsilicone) S4: Super X Clear (manufactured by Cemedine Co., Ltd.,one-liquid type specific modified silicone polymer) S5: Super X2 Clear(manufactured by Cemedine Co., Ltd., one-liquid type specific modifiedsilicone polymer)

EXPLANATION OF REFERENCE NUMERALS

-   1, 2, 3, 4, 5 shielding plate-   10 box-type structure-   20 opening-   21 joining structure-   22, 23, 24, 25, 26, 27, 28 shielding plate-   29 chipped off portion-   31 upper side of shielding plate-   32 lower side of shielding plate-   33 step-   41, 42, 43, 44, 45, 46 shielding plate-   51, 52, 53, 54, 55, 56 shielding plate

1. A shielding adhesive having neutron-shielding performance, whereinthe shielding adhesive contains a lithium fluoride powder having alithium fluoride purity of 99% or more.
 2. The shielding adhesiveaccording to claim 1, wherein the lithium fluoride powder has an averageparticle size of 2.4 μm or more and 5.2 μm or less.
 3. The shieldingadhesive according to claim 2, wherein the shielding adhesive is atwo-liquid curing type adhesive of which main component is an epoxyresin.
 4. The shielding adhesive according to claim 3, wherein theshielding adhesive contains the lithium fluoride powder in an amount of30 wt % or more and less than 60 wt %.
 5. The shielding adhesiveaccording to claim 2, wherein the shielding adhesive is a two-liquidcuring type adhesive containing an epoxy resin as a main component and amodified silicone resin as a curing agent.
 6. The shielding adhesiveaccording to claim 5, wherein the shielding adhesive contains thelithium fluoride powder in an amount of 30 wt % or more and 67 wt % orless.
 7. A method for repairing a defective part of shielding platesmade of a lithium fluoride sintered bod, comprising applying theshielding adhesive according to claim 1 to the defective part.
 8. Amethod for filling a gap at a joining portion of the shielding plates,comprising applying the shielding adhesive according to claim 1 to thegap.